While the demands concerning fuel efficiency and emissions are growing, mounting a cylinder internal pressure sensor to grasp a combustion state attracts attention. However, a cylinder internal pressure sensor is high in cost, and therefore, in order to adopt a cylinder internal pressure sensor, reduction in cost, and an additional merit such as substitution for another sensor are required.
As disclosed in, for example, Patent Literature 1, an internal combustion engine including a cylinder internal sensor is known. Further, the publication discloses a method which detects a heating value from the output value of the cylinder internal pressure sensor, and calculates an air-fuel ratio from the detected heating value in order to substitute the cylinder internal pressure sensor for an air-fuel sensor. More specifically, the publication discloses calculating an air-fuel ratio from a heating value/an injection time in a rich region (“/” is division sign. The same thing also applies to the following description), and calculating the air-fuel ratio from the heating value/the air amount in a lean region. According to the method like this, injection amount feedback control which corrects a fuel injection amount can be carried out based on the difference of the calculated air-fuel ratio and a target air-fuel ratio so that the air-fuel ratio corresponds to a target air-fuel ratio.